um: Fix get_signal() usage

If get_signal() returns us a signal to post
we must not call it again, otherwise the already
posted signal will be overridden.
Before commit a610d6e6 this was the case as we stopped
the while after a successful handle_signal().

Cc: <stable@vger.kernel.org> # 3.10-
Fixes: a610d6e6 ("pull clearing RESTORE_SIGMASK into block_sigmask()")
Signed-off-by: Richard Weinberger <richard@nod.at>